add stats for lossless

* Extend AuxStats with new fields
  it's slightly ABI-incompatible, but i guess it's ok for 0.1.99+
  I expect to add more stats later, possibly (predictor stats, etc.)
* Have cwebp report the features used by lossless
  compression (either for alpha or full lossless coding)
* Print the PSNR for alpha (useful in case of -alpha_q)
* clean-up alpha.c signatures
+ misc cleanup (added const '* const ptr', etc.)

Change-Id: I157a21581f1793cb0c6cc0882e7b0a2dde68a970

src/webp/encode.h

@@ -157,7 +157,7 @@ typedef struct WebPPicture WebPPicture;   // main structure for I/O
 typedef struct {
   int coded_size;         // final size
 
-  float PSNR[4];          // peak-signal-to-noise ratio for Y/U/V/All
+  float PSNR[5];          // peak-signal-to-noise ratio for Y/U/V/All/Alpha
   int block_count[3];     // number of intra4/intra16/skipped macroblocks
   int header_bytes[2];    // approximate number of bytes spent for header
                           // and mode-partition #0
@@ -173,7 +173,16 @@ typedef struct {
   void* user_data;        // this field is free to be set to any value and
                           // used during callbacks (like progress-report e.g.).
 
-  uint32_t pad[6];        // padding for later use
+  // lossless encoder statistics
+  uint32_t lossless_features;  // bit0:predictor bit1:cross-color transform
+                               // bit2:subtract-green bit3:color indexing
+  int histogram_bits;          // number of precision bits of histogram
+  int transform_bits;          // precision bits for transform
+  int cache_bits;              // number of bits for color cache lookup
+  int palette_size;            // number of color in palette, if used
+  int lossless_size;           // final lossless size
+
+  uint32_t pad[4];        // padding for later use
 } WebPAuxStats;
 
 // Signature for output function. Should return true if writing was successful.